Optional编程实例

 @Test
    public void optionalTest() throws NoSuchMethodException {
        String firstValue = null;
        Optional<String> key = Optional.ofNullable(firstValue);
        String defaultValue = key.orElse("apple");
        if (!key.isPresent()) {
            System.out.println(defaultValue + "001");
        }
        System.out.println(defaultValue);
    }

结果是 

apple001
apple

学习新的写法，新的api

=======================2021-11-12 20:29 =========================

public static class Apple {
        int size;
        int price;
        Color color;
        public Apple(int size , int price, Color color) {
            this.size = size;
            this.price = price;
            this.color = color;
        }

        public void setSize(int size) {
            this.size = size;
        }

        public Color getColor() {
            return this.color;
        }
    }

public static class Color {

        public Color(int color, Cat cat) {
            this.color = color;
            this.cat = cat;
        }
        int color;
        Cat cat;

        public Cat getCat() {
            return this.cat;
        }
    }

public static class Cat {
        int age;
    }

　　三个class层层嵌套

Apple e = new Apple(1, 1, new Color(1,null));
        //Apple e = new Apple(1, 1, null);
        Cat color = Optional.ofNullable(e).map(u -> u.getColor()).map(c -> c.getCat()).orElse(new Cat());

map方法返回值还是Optional，调用orElse进行取值